Methods of doping semiconductor materials and metastable doped semiconductor materials produced thereby

ABSTRACT

The structures of base semiconductor materials such as Si are modified by the use of isotope transmutation alloying. A radioisotope such as Si 31  is added into a base semiconductor material such as Si, and the radioisotope is transformed to a transmuted form within the crystal lattice structure of the base semiconductor material. A master alloy comprising a relatively large amount of radioisotope such as Si 31  may initially be made, followed by introduction of the master alloy into the base semiconductor material. When Si 31  is used as the radioisotope, it may be transmuted into P 31  within an Si crystal lattice structure. Metastable semiconductor materials doped with otherwise insoluble amounts of selected dopants are produced as a result of the transmutation process.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/331,718 filed May 4, 2016, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates to doped semiconductor materials, and moreparticularly relates to semiconductor materials such as silicon withdopants such as phosphorous that are formed by isotope transmutationprocesses.

BACKGROUND INFORMATION

Conventional semiconductor materials such as Si are doped with variouselements to change their charge carrier properties. For example, N-typedoped silicon materials may be coupled with P-typed doped siliconmaterials to form P—N junctions in devices such as solar cells. However,the types of elemental dopants that can be added to semiconductormaterials are limited due to factors such as insolubility. For example,as demonstrated by a conventional Si—P phase diagram, P is only slightlysoluble in Si at very high temperatures, e.g., 2.2 atomic percentsolubility at 1,131° C., and P is considered insoluble in Si at lowertemperatures such as room temperature.

SUMMARY OF THE INVENTION

The present invention provides methods for modifying the basic materiallattice structure of semiconductor materials at the atomic level. Oneembodiment of the process involves modifying the distribution and numberdensity of holes in these materials. The number density of holes isrelated to the efficiency of semiconducting properties such aselectrical conductivity. Thus, the present processes may be used tomodify properties of semiconductor materials.

In accordance with embodiments of the invention, the structure of a basesemiconductor material such as Si may be modified by the use of isotopetransmutation alloying. In this process, a radioisotope is added as analloying solute into the molten base solvent material. The radioisotopemay be selected such that solvent material properties are improved as aresult of the transformation of the radioisotope into its transmutedform after the base semiconductor material has solidified. This propertyimprovement results from the production of metastable thermodynamicstate due to the result of the transmutation process.

An aspect of the present invention is to provide a method of making asilicon-based semiconductor material comprising introducing Si³¹radioisotope into molten Si, solidifying the Si and Si³¹ radioisotope toform a transition material comprising atoms of the Si³¹ radioisotopewithin a Si crystal lattice structure, and P³¹ atoms retained in the Sicrystal lattice structure to thereby form a metastable silicon-basedsemiconductor material doped with P³¹.

Another aspect of the present invention is to provide a master alloy foruse in making a silicon-based material doped with P³¹, the master alloycomprising Si and Si³¹ radioisotope wherein the Si³¹ radioisotope ispresent in the master alloy in a greater amount than an amount of theP³¹ contained in the silicon-based material.

A further aspect of the present invention is to provide a metastablesilicon-based semiconductor material doped with P³¹ transmuted fromSi³¹.

These and other aspects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating methods of forming metastableSi doped with P in accordance with embodiments of the present invention.

FIG. 2 is an Si—P phase diagram illustrating insolubility of P in Si atroom temperature, and very limited solubility of P in Si within arelatively high temperature range.

FIG. 3 is a schematic diagram illustrating growth of a solid singlecrystal Si material from a molten bath of Si to which radioisotopematerial may be added in accordance with an embodiment of the presentinvention.

FIG. 4 is a schematic diagram illustrating growth of a solid singlecrystal Si material by a float-zone pulling process in whichradioisotope material is added in accordance with an embodiment of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating the production of metastableSi doped with P in accordance with embodiments of the present invention.As shown in FIG. 1, the radioisotope Si³¹ is introduced into a moltensemiconductor base material comprising Si. As used herein, “Si” meanssilicon in its standard isotope form, which predominantly comprises theSi²⁸ isotope. However, minor amounts of Si²⁹ and Si³⁰ isotopes may alsobe present in the Si. The Si³¹ radioisotope may be introduced in solidform into the molten Si. Alternatively, solid Si³¹ may be mixed withsolid Si, followed by melting of the mixture to thereby allow the Si³¹radioisotope atoms to diffuse in the molten Si material.

As further shown in FIG. 1, the Si³¹ radioisotope atoms easily diffuseinto the molten Si base semiconductor material at a self diffusion ratethat is the same for both Si³¹ and Si. The relative amounts of Si andSi³¹ may be selected such that essentially all of the Si³¹ radioisotopeatoms are soluble within the Si base semiconductor material at elevatedtemperatures and upon solidifying Si transforms to P and is frozen inthe lattice at room temperature both the elevated temperature of themolten Si and upon solidification and cooling, e.g., to roomtemperature.

As shown in FIG. 1, upon solidification, the Si³¹ atoms form a solidsolution in the crystal lattice structure of the Si base semiconductormaterial. As the Si³¹ radioisotope atoms transmute, they form P³¹ atomswithin the Si crystal structure. The Si³¹ thus transmutes to P³¹. Asolid material comprising Si and P³¹ is thereby formed in a metastablestate. As used herein, the terms “metastable” and “metastable state”mean a material containing both stable Si and Si³¹ radioisotope that issubsequently aged to transmute the Si³¹ to P³¹.

FIG. 1 also illustrates an alternative embodiment in which the Si andSi³¹ are initially solidified to form a master alloy. The Si/Si³¹ masteralloy may be added to an additional amount of molten Si to therebydiffuse the Si and Si³¹ atoms of the master alloy into the additionalamount of molten Si. Upon solidification, the cooled material comprisesSi³¹ radioisotope atoms from the master alloy, Si atoms from the masteralloy, and Si atoms from the additional amount of molten Si. The Si³¹radioisotope atoms then transmute into P³¹ atoms to form a metastablesolid material comprising P³¹ atoms within the Si crystal lattice.

In the embodiments shown in FIG. 1, the final semiconductor materialcomprises Si having a face centered cubic crystal lattice structure inwhich P³¹ atoms are substituted for Si atoms within the crystal latticestructure. The semiconductor material thus comprisesphosphorous-substituted silicon within a face centered cubic crystalstructure. In certain embodiments, the material may be provided insingle crystal form, while in other embodiments the material may bepolycrystalline. In certain embodiments, most or essentially all of theP³¹ atoms occupy sites within the Si crystal structure, rather thanforming as Si—P precipitates separate from the crystal structure. Thus,the majority of the P³¹ atoms are substituted into the Si crystalstructure rather than forming as precipitates. Typically, less than 10atomic percent of the P³¹ atoms form precipitates, for example, lessthan 5 atomic percent, less than 2 atomic percent, less than 1 atomicpercent, or less than 0.1 atomic percent, or less than 0.01 percent, orless than 0.001 percent.

The metastable Si and P³¹ semiconductor materials produced in theembodiments shown in FIG. 1 may include controlled amounts of P³¹. Incertain embodiments, the P³¹ may comprise from 0.001% to 10% atomicpercent of the Si/P³¹ material. For example, the P³¹ may comprise from0.0001% to 20% atomic percent, or from 0.001% to 2.2% atomic percent, orfrom 0.01% to 1.5% atomic percent. In a particular embodiment, when themetastable Si/P³¹ semiconductor material is used for solar cellapplications, the amount of P³¹ may comprise from 0.01% to 1% atomicpercent. Table 1 below lists the ranges above.

TABLE 1 Amount of P³¹ in Si Semiconductor Material (atomic percent)Ranges P³¹ Si A 0.00001%-20%   balance B 0.001%-2.2% balance C 0.01%-1.5% balance D 0.01%-1%  balance

To produce Si/P³¹ semiconductor materials as described above, the amountof Si³¹ radioisotope used to make such materials may correspond to theamounts of transmuted P³¹ described above. Thus, the amounts of Si³¹added to the molten Si base material may be within the same ranges asthe P³¹ amounts listed in Table 1 above.

When master alloys are formed, the P³¹ may comprise from 0.001% to 10%atomic percent of the Si/P³¹ material. For example, the P³¹ may comprisefrom 0.0001% to 20% atomic percent, or from 0.001% to 2.2% atomicpercent, or from 0.01% to 1.5% atomic percent.

Table 2 below lists ranges of Si³¹ radioisotope that may be used inSi/Si³¹ master alloys in accordance with embodiments of the presentinvention. In certain embodiments, the amount of Si³¹ in a master alloymay be up to 50%, for example, from 1 to 47%, or from 2 to 30%.

TABLE 2 Amount of Si³¹ in Si Master Alloy (atomic percent) Ranges Si³¹Si A 0.005%-20%  balance B 0.01%-10% balance C 0.05%-5%  balance E0.05%-2%  balance

In addition to Si and P³¹, the metastable Si-based semiconductormaterials produced in accordance with embodiments of the presentinvention may further include minor amounts of other elements such asGe, Ga, As, B, Al, Se and the like.

The Si³¹ radioisotope material may be produced by bombarding Si withhigh energy neutrons to create in-situ Si³¹, for example, at roomtemperature or by bringing the Si up to a temperature close to themelting point of Si, e.g., either above or below the Si melting point.The neutron flux used during bombardment of the Si may typically rangefrom 10¹⁰ to 10¹⁶ neutrons/cm·sec, or from 10¹¹ to 10¹⁵neutrons/cm²·sec, or from 10¹² to 10¹⁴ neutrons/cm²·sec. The neutronbombardment may be carried out for a suitable period of time, forexample, ranging from 1 second to 72 hours, or from 30 seconds to 48hours, or from 1 minute to 24 hours, or from 30 minutes to 12 hours.

Radioisotope Si³¹ material produced as described above may be added tothe base Si semiconductor by adding the isotope Si³¹ directly intomolten Si, or by using a seed Si³¹ material or master alloy, e.g., togrow single crystal Si in floating zone equipment. These methods may beset up in solid form before melting.

When an Si/Si³¹ master alloy is used, it may be made in a similar manneras described above, except higher concentrations of Si³¹ may be presentin the master alloys, e.g., as described in Table 2 above.

Solidification of the molten Si/Si³¹ material may be performed at slowcooling rates, e.g., 50° C./min to room temperature.

Transmutation of the Si³¹ to P³¹ within the solid Si material occurs insitu by aging the material for an appropriate amount of time. Duringaging, conversion of Si³¹ to P³¹ occurs within the Si lattice, which isretained in the same crystal structure.

Once the metastable Si and P³¹ material has been formed, it may befabricated into any suitable type of semiconductor device. For example,the metastable Si/P³¹ material may be used as N-type materials in P—Njunctions of semiconductor devices such as solar panels.

Although the Si³¹ radioisotope is primarily described herein, it is tobe understood that other types of radioisotopes may be generated andused in accordance with embodiments of the invention. In accordance withembodiments of the present invention, radioisotopes such as Si³¹described above may be selected based on specific criteria: theradioisotope should have solubility in the base material; the transmutedelement derived from the radioisotope is insoluble or has limitedsolubility in the base semiconductor material; the radioisotope has ahigher diffusion rate in the base semiconductor material than thetransmuted element; the transmuted element derived from the radioisotopehas a low diffusion rate in the base material; the radioisotope isselected based on the atom size of the transmuted element such that itinduces atomic size mismatch stresses in the crystal lattice; theradioisotope is selected based on the position it takes in the crystallattice before transmutation, e.g., a substitutional site or aninterstitial site (octahedral or tetrahedral); the radioisotope isselected based on its cost, ease of manufacture and abundance; theradioisotope is selected based on its half-life; the radioisotope isselected based on energy imparted into the base material on decay; andthe radioisotope is selected based on the need of the end pointapplication.

Examples of radioisotopes selected for various types of basesemiconductor materials in accordance with embodiments of the inventionare listed in Table 3.

TABLE 3 Radioisotopes and Transmuted Elements Semiconductor Isotope HalfLife Transmuted Element Si Si³¹ 2.62 hours P³¹ Ge—Ga Ge⁷¹ 11 days Ga⁷¹Si—Ge Ge⁷¹ 11 days Ga⁷¹ Ge—Se Se⁷³ 120 days As⁷⁵

While the present invention may include additional combinations of basesemiconductor materials, radioisotopes and transmuted elements, thepresent description is primarily directed to a silicon-basedsemiconductor material comprising Si doped with P³¹ transmuted from S³¹radioisotope. However, it is to be understood that other combinationsare within the scope of the present invention. For example, the methodcan be extended to add any other isotope that will give as donor or asacceptor element to Si.

Consider a pure semiconductor like Si. The element P when added gives adonor impurity. Phase diagrams for Si—P indicate that P is soluble atabout 2.2 atomic percent at 1,131° C., and is insoluble at roomtemperature. With this limited solubility, only limited numbers of holescan be produced. However, if the Si³¹ radioisotope is doped into Si, itis possible to adjust to higher/lower number densities of holes.

The effect of adding radioisotope of Si³¹ (solute) to thenon-radioactive pure Si (solvent). Si³¹ has a half-life of 2.62 hoursand will decay to a transmutation product P³¹ emitting β⁻radiation. Ifsuch an element Si³¹ is added to a molten pure Si and the resultingalloy is cooled to room temperature the atoms will randomly distributein the lattice of Si. These atoms will transmute in-situ into the atomsof the transmuted product P³¹. Thus the transmuted product will befrozen in the lattice of Si. This method of forming a metastable solidsolution of P in Si can exceed the equilibrium solubility of the alloydictated by Si—P phase diagrams.

It is possible to estimate the charge carrier concentration andconductivity of Si doped with P (same as P³¹) 0.1% at (P-concentrationC_(P)=10⁻³) amount at room temperature. For this calculation we definethe following parameters:

-   -   For solvent Si: density ρ=2330 kg/m³; at.wt.=28; lattice        parameter a=5.43×10⁻¹⁰ m; atomic volume of a unit Si        cell=a³=1.6×10⁻²⁸ m³; electron mobility μ_(e)=0.15 m²/V−s; hole        mobility μ_(h)=0.05 m²V−s; energy gap E_(g)=1.2 eV;    -   Proton charge e=1.6×10⁻¹⁹ C, Avagadro's # A=6.02×10²³    -   N_(P)=charge carrier concentration; N_(Si)=# of Si atoms per        unit volume; n_(c)=concentration electron charge in conduction        band; n_(v)=concentration of holes in valance band.    -   N_(Si)=(8 atom in unit cell)/(cell volume)=8/1.6×10⁻²⁸=5×10²⁸/m³        N_(P)=N_(Si)×C_(P)=5×10²⁸×10⁻³=5×10²⁵/m³=charge carrier        concentration n_(c), neglecting very small n_(v).        For intrinsic semiconductors like Si, n_(c)=n_(v).        With complete ionization of P in Si, the charge carrier        concentration        n_(c)=N_(P)=5×10²⁵/m³        for P=10⁻³ concentration as an example.

We now calculate the conductivity (a) of Si/P semiconductor:σ(Si/P)=N_(P) eμ _(e)=(5×10²⁵/m³)×(1.6×10⁻¹⁹C)×(0.15m²/V−s)=1.2×10⁵/ohm·m

The intrinsic carrier concentration (n_(i)) in pure Si refers toelectron (or hole) concentration. Commonly accepted values of n_(i) forSi is about 1.4×10¹⁵/m³. This value is significantly smaller than forSi/P n_(c)=5×10²⁵/m³ for P=0.1 at % concentration, by about 10¹⁰.

Intrinsic Si conductivity σ_(Si)=n_(i)·e[μ_(e)+μ_(h)], since n_(c)=n_(v)for pure Si; hence:σ_(Si)=1.4×10¹⁵×1.6×10⁻¹⁸×0.2=0.448×10⁻³=4.5×10⁻⁴/ohm·mThis number is fixed for Si.

This conductivity of Si is significantly smaller by about 10⁹ comparedto Si/P alloy at a P=10⁻³ concentration.

Thus the carrier concentration and its conductivity can be estimated,with the assumption of complete ionization of P in Si. The calculationsuggests that isotope alloying of a semiconductor Si can besignificantly improved with very small concentration of dopingimpurities like P. Deliberate additions of impurities to a solventmaterial in a controlled manner can allow charge concentrations to betailored in order to improve conductivity to desired values needed forspecific applications.

In accordance with certain embodiments, metastable silicon-basedsemiconductor material may typically have a conductivity of from 10⁵ to10¹⁷ per ohms-m, for example, from 10⁷ to 10¹⁵, or from 10⁹ to 10¹³, orfrom 10⁹ to 10¹².

In accordance with certain embodiments, metastable silicon-basedsemiconductor material may typically have a carrier concentration offrom 10²⁰ to 1040/m³, for example, from 10²⁷ to 10³³, or from 10²⁹ to10³⁴, or from 10²⁸ to 10³⁴.

Various methods can be used to add the alloying radioisotope into thebase metal solvent. These include: directly add the isotope into tomolten base material, solidify and fabricate; surface coating of theisotope onto the base material and diffusing it into the base materialat a desired temperature; thermal spray on the surface of the basematerial and diffuse it in; add the isotope to the seed crystal to growthe single crystals; powder blends of isotope with the desired basematerial, compact, sinter into a final wrought product; and mechanicallyalloy via ball milling method both the isotope and the base material.

The following example is intended to illustrate various aspects of thepresent invention, and are not intended to limit the scope of theinvention.

EXAMPLE

An amount of Si is bombarded by a neutron flux at an energy level offrom 10¹² to 10¹⁴ neutrons cm²·sec for a sufficient amount of timewithin 1 to 12 hours to change a desired percentage of the Si atoms toSi³¹ radioisotope atoms, e.g., up to 50% conversion of the Si to Si³¹.Bombardment with the neutron flux is conducted at any suitabletemperature, including room temperature, or close to the melting pointof Si, to form a master alloy. The resultant master alloy, in eithersolid or molten form, is added to base Si material, e.g., in a moltenstate. Upon cooling, a metastable material is formed comprising astandard Si crystal lattice containing P³¹ atoms that have beentransmitted from the S³¹ radioisotope atoms.

In one embodiment of the invention, radioisotopes are added tosemiconductor materials used to make photovoltaic cells. Theradioisotope is selected based on its solubility and diffusivity in thesemiconductor and based on the doping effectiveness of its transmutatedproduct. Semiconductor materials may be N-type materials used to make upP—N junctions used in solar cells. For example, transmutation of Si³¹ toP³¹ over time results in increased efficiency of the photovoltaic cells(or other semiconductor devices).

In one example of the embodiment, a uniform distribution of phosphorousis formed in the lattice of silicon which may be used to make the N-typematerial in a P-N junction in a solar cell. Prior work showed thattechniques such as diffusion do not provide a uniform distribution ofdopant in silicon. More uniform distribution of dopants could beachieved using techniques such as neutron transmutation doping (NTD) butthis technique requires that the material be exposed to a neutron fluxwhich induces extensive damage to the silicon lattice.

In this example of the embodiment, the radioisotope Si³¹ is mixed in apredetermined ratio with the Si in the molten bath used to grow theN-type silicon material. The Si radioisotope is totally soluble and willself-diffuse within the silicon lattice. Thus on solidification the Si³¹will be randomly distributed in the lattice. The Si³¹ in the latticedecays to form P³¹.

Thus, N-type silicon material with random-uniform distributions ofphosphorous present in the lattice at predetermined concentrations canfabricated. Very little segregation of phosphorous is observed on grainboundaries if polycrystalline silicon is grown. Phosphorousconcentrations of 10¹³ to 10¹⁸ atoms/cm³ may be achieved using theapproach. Thermal annealing treatments may not be required due to theuniform distribution of the dopant and the relative lack of damageinduced by the process.

The N-type silicon fabricated as per this example may be coupled with aP-type silicon material to form a P-N junction in a solar cell device.

The disclosed process can be used to improve performance ofsemiconductor materials used in various applications. For example theefficiency of the following components can be improved: solar cells toimprove its efficiency; metal oxide semiconductor chemical sensors;silicon crystals used in CPU; and high power electronic devicecomponents

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

What is claimed is:
 1. A method of making a silicon-based semiconductormaterial comprising: introducing a master alloy comprising Si and Si³¹radioisotope into a base Si material to form a transition materialcomprising atoms of the Si³¹ radioisotope within a Si crystal latticestructure; and aging the transition material to convert at least aportion of the Si³¹ radioisotope atoms to P³¹ atoms retained in the Sicrystal lattice structure to thereby form a silicon-based material dopedwith P³¹.
 2. The method of claim 1, wherein the amount of P³¹ in thesilicon-based semiconductor material is above an equilibrium solubilitylimit of phosphorous in silicon as determined by a standard Si—P phasediagram.
 3. The method of claim 2, wherein the equilibrium solubilitylimit is a room temperature solubility limit.
 4. The method of claim 1,wherein the amount of P³¹ retained in the Si crystal lattice structureis from 0.0001 to 40 atomic percent of the Si crystal lattice structure.5. The method of claim 1, wherein the amount of P³¹ retained in the Sicrystal lattice structure is at least 0.001 atomic percent of the Sicrystal lattice structure.
 6. The method of claim 1, wherein the amountof P³¹ retained in the Si crystal lattice structure is from 0.01 to 30atomic percent of the Si crystal lattice structure.
 7. The method ofclaim 1, wherein the master alloy is introduced into molten base Simaterial.
 8. The method of claim 7, wherein the master alloy isintroduced at a temperature above room temperature into the molten baseSi material.
 9. The method of claim 1, wherein the master alloycomprises from 0.005 to 50 atomic percent Si³¹ radioisotope.
 10. Themethod of claim 1, wherein the master alloy comprises from 0.05 to 40atomic percent Si³¹ radioisotope.
 11. The method of claim 1, wherein theSi crystal lattice structure is face-centered-cubic, and the P³¹ atomsrandomly occupy sites within the face-centered-cubic structure.
 12. Themethod of claim 1, wherein the silicon-based semiconductor material hasa conductivity of from 10⁵ to 10¹⁷ per ohms·m.